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Mud Pumping Diagnosis and Treatment

P Sharpe, J Herbert

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Mud Pumping Diagnosis and Treatment 

GEOfabrics Limited. Skelton Grange Road, Stourton, Leeds, West Yorkshire. UK

Keywords: Geotextiles, Filtration, erosion pumping, sand blanket

ABSTRACT: Mud pumping caused by subgrade erosion under ballasted railroad is a well known phenomenon and has been the subject of much research. The effects it can have on track stability and maintenance costs are significant so understanding the symptoms to allow appropriate treatment is critical. This report describes the indicators and investigative techniques used to identify where mud-pumping  is present  in trackbed.

Ballast filled with mud is significantly weakened

Introduction

Since the start of the 21st century there has been a growing interest in the phenomenon of mud pumping, and the effects it can have on track stability and maintenance costs.  The symptoms of mud pumping are similar to those of life expired ballast, i.e. wet spots appearing at the surface of the ballast accompanied by rapid deteriorating of track geometry.  However, from a track management perspective there are enormous differences.   If the mud pumping problem is not correctly treated at the time of ballast renewal the life of the trackbed can be less than 100MGT, compared to 1000MGT for ballast which deteriorates gradually under the action of trafficking and repacking.  Unfortunately, effective treatment of mud pumping can significantly increase the cost of ballast renewal.  It is therefore essential to correctly identify mud pumping boundries and minimize the length of treatment required.

What causes mud pumping?

Mud pumping is characterized by fine silt or clay particles migrating upwards through the ballast layer in the form of a viscous slurry, and is often associated with poor drainage.  In most roadbeds, particularly those having a clay or silt subgrade, an interlayer of crushed, well graded aggregate would have been placed over the subgrade at the time of construction to separate it from the ballast.  However, over time  the interlayer may have degraded/weathered or been inadequate to protect the subgrade, especially under today’s heavy axle loads.   The fine particles which result in mud pumping can therefore derive either from the subgrade or degraded/weathered interlayer.  In either case, the fine particles combine with water to produce a viscous slurry which is highly mobile. The cyclic action of passing traffic creates a pumping action which squeezes this slurry into the voids within the ballast, reducing its bearing capacity and inhibiting drainage, leading to rapid track settlement and deterioration in track geometry.  In its advanced stages the track becomes impossible to maintain to the required standard, in which case slow orders have to be imposed.

 Identification of Mud Pumping

Before designing remedial treatments for a roadbed that has become heavily contaminated with fines it is important to confirm whether a significant amount of the fines have migrated upwards from the interlayer. There are a number of ways in which ballast can become contaminated with fines which can be broadly classified either “bottom up” or “top down” contamination.   Bottom up contamination is essentially mud pumping, while top down contamination includes fine material generated by crushing and abrasion of the ballast over time, wind-blown fine sand or silt, material dropped from open trucks or from overtopping of embankments in poorly drained areas.  Effective remediation of bottom up  contamination will inevitably involve treatment of the interlayer.

Identifying sources of fines

When contamination appears at the surface of the ballast it is not necessarily a cause for immediate concern, provided it does not affect track geometry.  However it is important to important to identify the source of contamination as accumulation of fine material resulting from top down contamination, can reduce the permeability of the ballast to such an extent that wet spots develop.  These can look similar to mud pumping, and if left untreated can have a similar impact on track geometry to mud pumping.

It is important to note the difference between mud pumping and problems that sometimes occur due to ballast degradation as it reaches the end of its life, particularly if the derainage is poor.  The most obvious clue is sometimes a marked difference in colour between the ballast and the slurry, as shown in  Img 1. Img 2 shows a wet spot caused by life expired ballast.

IMG 1 Mud pumping over a red mudstone subgrade
IMG 2 Wet spot formed by degraded ballast

The next two photographs below show elevations from trial pits excavated at the side of the track to show the condition of the roadbed.

Img 3 shows a developing mud pumping problem. Just two years after ballast renewal the geotextile separator has failed and allowed fines to migrate upwards from the interlayer.

Img 4 shows a life expired ballast layer over an effective interlayer. The ballast around the tie appears completely filled with fine material, yet there Is no intermixing of the ballast and interlayer.

IMG 3 fine material migrating upwards from the interlayer
IMG 4 Life expired ballast

Early signs of mud pumping

There are many areas where mud pumping problems occur frequently and these are usually well known to the track engineers, but it is likely that mud pumping occurs in other areas, yet it has never been recognised as such.

A small of clay slurry at the base of the ballast does not affect bearing capacity of the roadbase.  However if the slurry migrates upwards the rate at which track geometry deteriorates will increase.  The deterioration of geometry usually becomes evident to the track maintenance teams long before the slurry appears at the surface, by which time the track is virtually unmaintainable, particularly on a heavily loaded line.

An increase in rate of deterioration is always a cause for concern.  At this point a visual inspection is recommended to see if there are any obvious causes of worsening geometry which are not related to interlayer condition, such as unstable earthworks, which can be addressed separately.  If there are no obvious causes it is useful to undertake a search of existing information to assess the likelihood of the problem being due to bottom up contamination.  The following sections summarise the types of information which may be available to the track engineer.

Geometry Problems

Most railroads are monitored frequently to assess the roughness of the track and determine the rate of deterioration of the track geometry, primarily to schedule track surfacing operations.

However, the characteristics of the track roughness and the growth of localized faults in track alignment can be good indicators of where there are problems with the roadbed.  For example the alignment problems shown in Img 5 were caused by mud pumping, subsequently confirmed by geotechnical investigation, but for the most part there is no visual evidence at the ballast surface.  This indicates that mud pumping can cause poor track geometry long before there is visual evidence at the surface.

Img 5 Deterioration of track geometry can be a tell tale sign

Geology/Hydrogeology

Geology is often an important contributory factor in the development of a pumping problem.  For example which, if a naturally stiff clay/silt is not protected by an adequate interlayer it will degrade rapidly under dynamic loading in the presence of water.  Clays are naturally of low permeability, so drainage of the interlayer is an important consideration.  As a result it is common to find mud pumping in a clay cut.  There are also many mud pumping problems on embankments formed from clays; the centre of the embankment becomes compacted with time due to the passage of trains, rendering it impermeable, at the same time forming a trough along the embankment, which is difficult to drain.

Other types of subgrade are highly permeable, such as fissured rocks or sandy soils, which can often support the ballast without an interlayer.  If the groundwater is well below the track the roadbed is free draining and therefore provides ideal track support.

Geological and hydrogeological records are available in many areas, but may require interpretation together with track drainage provisions by a specialist in order to assess whether a particular area is liable to mud pumping.

Ground Penetrating Radar (GPR) survey

One of the best techniques for observing the development of mud pumping is ground radar.  Radar waves generated from above pass easily through clean ballast without any reflection but are slowed down when they encounter wet or fine-grained material.  The interface between clean ballast and wet/fine material is represented on the GPR plot shown below  in Img 6 by the undulating double black and white line on the grey background.  Rising mud is usually characterized by a strong shallow irregular interface.  A trace from the track geometry car is also shown to highlight problem areas.  The correlation between rough track and shallow interface is obvious.

Img 6 The double black and white line on the grey readout shows the interface between clean ballast and fine material

Geotechnical Investigation (GI)

If visual inspection and desk study of available information indicate possible mud pumping it is important to undertake some form of roadbed GI .  Shallow hand excavated trial holes are normally adequate to confirm whether fines are migrating upwards from the interlayer, although ideally deeper investigation, typically down to 3ft below top of rail, should be undertaken for the design of suitable remedial measures.  Trial holes should be inspected and logged by a track/geotechnical engineer with relevant roadbed experience.

In areas with poor track drainage or high water table hand dug trial holes may not be adequate as it is not possible to log materials underwater.  Window-sampling in the roadbed is a possible option in this case.   By way of illustration the photographs shown below are of window sample cores, split to show texture, from three different sites.  The cores in Img 7 show samples taken a site in Pueblo, with a good quality interlayer separating ballast and subgrade.  Img 8 shows two sites from the UK, Img 8a showing fines rising through a poor quality interlayer from the underlying clay subgrade. Img 8b shows a failed interlayer.

Img 7 Cores showing good quality interlayer separation
Img 8a fines rising through a poor quality interlayer
Img 8b failed interlayer

Recommendations for Interlayer

Remedial Measures

Table 1 gives the recommended treatments to remediate problems with the interlayer for most commonly encountered roadbed conditions.  It assumes that the existing interlayer remains substantially intact, i.e. it would continue to perform in the same way as it has done in the past if the only treatment to be undertaken is reballasting.

For completeness, Table 1 includes the ‘no-treatment required’ option, for the case that the existing interlayer consists of clean, well graded aggregate with no evidence of bottom up contamination. It also addresses situations where there is a possibility of minor upwards migration of fines intermixing with the base of the ballast layer from the interlayer; in such situations the inclusion of a separating geotextile will give some increase in ballast life.

Drainage considerations

The performance of an interlayer can be affected by the quality of drainage on a site.  Poor drainage can, to some extent, be compensated by adopting a more robust interlayer treatment.  The following classifications offer general guidance to describe the quality of drainage at a particular location.

Poor Drainage

Site has a history of drainage problems, e.g. standing water identified during site investigation within 0.5m of the base of the tie. Existing drainage system is not functioning

Satisfactory Drainage

No history of drainage problems, e.g. water level identified at 0.5m below base of tie during site investigation or lower.  Existing track drainage is functioning well. Existing subgrade is not free draining.

Good Drainage

Site has no history of drainage problems, no standing water identified during site investigation.  Existing subgrade is identified as free draining.

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